The present invention relates to a control circuit for a generator, a generator assembly and methods for controlling the output voltage of a generator.
Generators are typically a primary electrical power source in a plurality of vehicles (automobiles, vessels, aircrafts). In the following, for illustration purposes, the invention will be described with reference to an automotive electric generator like, for instance, a Lundell machine. However, the invention is not so limited and may find its application in conjunction with any other type of generator.
One of the challenging problems regarding generators which provide electric power for a vehicle electrical system relates to the instability of the vehicle electrical system voltage, in particular in the case of sudden variations of the load of the generator. In this context, sudden variations of the load of the generator may relate to sudden load increases as well as sudden load decreases, which are also referred to as load dumps.
Conventionally, these load dumps require extensive precaution measures to accommodate the resulting voltage variations. In particular, compared to the requirements without consideration of these—partly extreme—voltage variations, the load dumps lead to an over dimensioning of the protective components and an over dimensioning of the breakthrough voltages of all (semiconductor) components which are connected to the vehicle electrical system.
For instance, for the 14 V vehicle electrical system of an automobile, typically semiconductors with a breakthrough voltage of 55 V to 60 V are required. FIG. 1 illustrates an overview of the specified voltages and voltage ranges related to a vehicle electrical system. The diagram in FIG. 1 illustrates that the maximum voltage Uloaddump during a load dump may be specified for example as high as 45 V.
Referring to one example of a conventional generator assembly as illustrated in FIG. 2, a generator assembly 10 for a vehicle electrical system with a field regulator 11 is illustrated. The field regulator 11 used in the conventional generator assembly 10 of FIG. 2 is also known as a single quadrant chopping circuit or chopper because a current if through a excitation coil 12 (also referred to as field current) and the corresponding excitation coil voltage uf (also referred to as field voltage) are either positive or zero at any given time. To simplify matters, the excitation coil 12 can be referred to by its inductance Lf.
The generator assembly 10 also includes a generator 15 (e.g., a Lundell alternator, also known as claw pole alternator), which may be a wound-field, 3-phase synchronous machine. The corresponding three output terminals of the generator 15 are coupled to a rectifier 16, e.g., a 3-phase avalanche bridge, to provide a rectified generator output voltage UBN to a vehicle electrical system.
In today's generator assemblies, it is typically merely the rectified generator output voltage UBN across the “+” and “−” terminals of the generator assembly which is measured, conditioned by a signal conditioning block 21 and compared with a rectified generator output voltage setpoint UBN—ref (also referred to as reference voltage) within a generator controller 20. The signal conditioner 21, e.g., a filter with appropriate bandwidth, provides a feedback signal which is subtracted from the reference voltage UBN—ref. The resulting difference signal is received as input by a voltage controller 22. The voltage controller 22 controls a pulse width modulation (PWM) generator 23 that modulates the pulse width of a MOSFET (power) transistor T1 via a driver circuit 24, wherein the transistor T1 is in series with the excitation coil 12.
The duty cycle of the MOSFET transistor T1 is controlled such that the excitation coil current if maintains the rectified generator output voltage UBN at a desired level for a given rotational speed of the generator 15 and load 30 of the generator assembly 10. I.e. the excitation coil current if is the actuating variable of a control loop to control the rectified generator output voltage UBN. A freewheeling diode D1 provides a freewheeling path for the excitation coil current if during the periods when the MOSFET transistor T1 is switched off.
The corresponding control process within a conventional generator controller can be characterized as follows: If the rectified generator output voltage UBN is lower than the reference voltage UBN—ref, the duty cycle of the driving circuit 24 for field regulator 11 of the excitation coil 12 is increased. In case the rectified generator output voltage UBN is higher than the reference voltage UBN—ref, the duty cycle of the driving circuit 24 for the excitation coil 12 is reduced to zero. Hence, the degaussing of the excitation coil 12 is effected by the freewheeling diode D1.
Further with regard to FIG. 2, during a load dump, the generator assembly 10 is abruptly disconnected from the battery 40 and all or most part of the load 30. Consequently, the excitation coil current if has to be reduced from a relatively high value needed for a high load to a relatively low value or even zero needed for the reduced or completely missing load.
Accordingly, the energy stored in the field of the excitation coil 12 causes a strong transient of the vehicle electrical system voltage UBN across the terminals “+” and “−” of the generator assembly 10 during a dump of the load 30. When a normal rectifier 16 is used, the peak of the voltage transients may reach levels multiple times the nominal generator output voltage Unom. Moreover, the duration of the voltage transients may last in the order of several hundred milliseconds prior to dropping below a maximum specified generator voltage Unom,max.
The peak transient generator output voltage may be clamped to acceptable levels by using, for example, avalanche rectifiers for the rectifier 16 which may absorb the excess generator output power until the magnetic field of the excitation coil 12 decays to lower levels. In a conventional generator assembly, the duration of a transient caused by a load dump depends—to the greatest extent—on the field time constant which results from using the field regulator 11 to control the current if through the excitation coil 12. In the case of the example in FIG. 2, the excitation coil current if as cause of magnetic field of excitation coil 12 decays at its natural rate through the freewheeling diode D1 when the MOSFET transistor T1 is turned off to disconnect the excitation coil 12 from the battery 40 in order to initiate the decay of the excitation coil current if.
Not only does the load dump energy cause a need for over dimensioning of protective components for the devices connected to the generator; in fact, since the load dump energy has to be absorbed by e.g., the avalanche rectifiers, it may require much larger devices or several devices of the rectifiers in parallel to secure reliable operation. This particularly holds for higher power generators. Hence, the above mentioned devices significantly increase the cost, size and weight of a generator which, in particular for vehicle applications, is highly undesirable.
For these or other reasons, there is a need for the present invention.